Coating aluminum



Patented July 10, 1934 PATENT OFFICE COATING ALUMINUM Harold K. Work,Oakmont, Pa., assignor, by

mesne assignments, to Aluminum Colors, Incorporated, Indianapolis, Ind.,a corporation of Delaware No Drawing. Application January 26, 1932,

' Serial No. 589,056

8 Claims.

' This invention relates to oxide-coated aluminum by which termis'herein defined aluminum and aluminum base alloys coated with a hardadherent coating of appreciable thickness composed in substantial partof aluminum oxide.

The so-called processes of oxide-coating aluminum, which consistessentially in providing aluminum with a hard adherent coating composedin substantial part of aluminum oxide, have become a matter ofconsiderable commercial import. Among such processes, the one findingwidest application is based upon the use of sulfuric acid solutions asthe coat-forming electrolyte. The process comprises making the aluminuman anode in an electrolytic cell the electrolyte of which is a solutioncontaining up to about per cent of sulfuric acid by weight. About 5 to40 volts are impressed upon the cell at temperatures usually below about40 centigrade. Under the combined action of the acid and the current, anoxide coating, the specific characteristics of which depend insubstantial part on the concentration of the electrolyte, is formed onthe aluminum. While such processes have been considered to be generallysatisfactory, they have certain inherent disadvantages which are noteasily overcome.

The general object of this invention is the improvement of the abovemethods by the provision of a modified sulfuric acid solutionelectrolyte which is capable of producing coatings of improvedproperties and which is, moreover, excellently adapted to commercialoperation. Further and similar objects will appear in the followingdescription of my invention.

I have discovered that the action of sulfuric acid solution electrolytesin the building up of oxide coatings on aluminum may be considerablymodified and improved in operation when there is mixed with suchsolutions a dib'asic organic acid. When an electrolyte containing amixture of sulfuric acid and dibasic organic ac d is used in connectionwith the processes of oxide coating above described, the coatingsobtained havethe favorable properties of coatings produced in sulfuricacid solution electrolytes but many of the disadvantageous properties ofsuch coatings are rectified. Moreover, the electrolyte is more amenableto commercial operation than is sulfuric acid electrolyte. An example ofa property of the oxide coating which is favorably influenced by the useof a sulfuric acid-dibasic organic acid electrolyte in the formation ofthe coating is the abrasion resistance. One of the common reasons foroxide-coating aluminum is to produce a product having a considerablygreater resistance to abrasion than do plain aluminum surfaces. Inoperation with sulfuric acid electrolytes, it is requisite that thesolutions be maintained at temperatures below about 30 centigrade lestthe coatings tend to become soft or powdery and the abrasion resistancefall sharply. Therefore, when coating large surfaces, where the powerinput is substantial, artificial cooling of the electrolyte is necessaryif the electrolyte temperature is to be kept low enough to give coatingsof satisfactorily high abrasion resistance. Where cold tap water is notavailable, artificial refrigeration may have to be installed and theattendant expense adds substantially to the cost of operation. Thematter of refrigeration has been a considerable commercial drawback inthe coating of aluminum in sulfuric acid electrolytes. However, when, inaccordance with my invention, an electrolyte of a mixture of sulfuricand dibasic organic acids is used, the necessity for the use ofrefrigeration is substantially eliminated because not only do thesulfuric acid-dibasic' organic acid electrolytes produce oxide coatingsof higher abrasion resistance than does the sulfuric acid electrolyte,but my new electrolyte, likewise, will produce coatings of high abrasionresistance at comparatively high temperatures.

A standard test for the abrasion resistance of such coatings is carriedout by rotating a plate of the oxide-coated aluminum against an abrasivewheel under a constant pressure. The number of revolutions of the platerequired to penetrate the coating is taken as a measure of the abrasionresistance. Applyingthis test to samples of oxide-coated aluminum whichhave been prepared bymaking the aluminum an anode in an electrolyticcell the electrolyte of which was composed of a solution containing 25per cent by weight of sulfuric acid, and to samples prepared by makingthe aluminum the anode in an electrolytic cell the electrolyte of whichwas composed of a solution containing3 per cent by weight of sulfuricacid and 3 per cent by weight of oxalic acid, the following results wereobtained. Operating the cells at a temperature of about 25 centigrade,the abrasion resistance of the oxidecoated aluminum which was preparedin the sulfuric acid electrolyte was 340 while the abrasion resistanceof the oxide-coated aluminum prepared in the sulfuric acid-dibasicorganic acid electrolyte was 490. Operating the cell at a temperature ofabout 37 centigrade, the abrasion resistance of the oxide-coatedaluminum produced in the sulfuric acid electrolye was only 50 while theabrasion resistance of the oxidecoated aluminum produced in the sulfuricacid-. dibasic organic acid electrolyte was 580. These 1 figures are butexemplary of the effect produced by the use of electrolytes of varioussulfuric acid concentrations and by the use of electrolytes containingvarious amounts of sulfuric acid and dibasic organic acid.

The oxide-coatings produced by the use of sulfuric acid-dibasic organicacid electrolytes in accordance with my invention retain the desirablecharacteristics of the oxide coatings produced in sulfuric acidelectrolyte. white; they are hard; they are composed in substantial partof aluminum oxide;

providing it with a protective coating of great practical value and ofconsiderable beauty.

It is of course necessary, in making the sulfuric acid-dibasic acidelectrolytes, to use a di basic organic acid which, under the conditionsnamed, is stable and is soluble. Among such acids '1 have found thatcertain are to be preferred and that, although the advantages of myinvention may be realized with the other dibasic organic acids, andoften to a very useful end, oxalic acid (112C204), malic acid (HsC4O5),and

' malonic acid (C3H4O4), when mixed with sulfuric acid to form theelectrolyte, produce coatings in which appear to be realized thegreatest benefits of my invention. Among other dibasic acids which areuseful are maleic acid (114C404), succinic acid (C4H6O4), and so forth.The dibasic organic acid may be mixed with sulfuric acid in varyingproportions and excellent oxide coatings produced, by the use thereof,on aluminum. Concentrations of sulfuric acid as high as 70 per cent byweight may be used, and'in the practiceof my invention I have found thatcommercial electrolytes containing from about 0.5 to 15 per cent byweight of sulfuric acid and about 0.5 per cent by weight up to about thesaturation limit (usually about 9 per cent by weight) of oxalic acid areparticularly advantageous in large scale operation. For instance, anelectrolyte containing 0.5 per cent by weight of sulfuric acid and 3 percent by weight of oxalic acid has given very good results as haslikewise an electrolyte containing 9 per cent by weight of sulfuric acidand 5 per cent by weight of oxalic acid. Also an electrolye containing 3per cent by weight of sulfuric acid and 3 per cent by weight of malicacid has been found very suitable for operation, as

.has also a solution containing 3 per cent by weight of sulfuric acidand 3 per cent by weight of malonic acid.

In the practice of my invention the sulfuric acid-dibasic organic acidelectrolyte is placed in an electrolytic cell the cathode of which maybe any suitable material such as aluminum or lead and the anode of whichis the aluminum or aluminum alloy article which is to be coated. Avoltage, which is usually about 10 to 30 volts but which may be as highas 50 volts, is impressed upon the cell and the operation is continueduntil a coating of the desired thickness is formed.

desirable and since the thickness is a direct function of the time oftreatment, it is a desirable The coatings are and they strongly adhereto the surface of the aluminum,-.

property of my electrolyte that thick coatings,

hard of surface, may be built up on aluminum.

Although I have referred to electrolytes made of solutions of sulfuricand dibasic organic acids,

'I have intended by this designation to include acid solutions of thesalts of sulfuric acid and dibasic organic acid when those salts aresuch that their combined presence in solution does not precipitate oneor another of the acid radicals and when the metals from which suchsalts are formed do not tend to plate out of solution upon the cathodebeing treated. Such, for instance, are the salts of the alkali metals,including ammonium, and acid solutions of these salts are within thepurview of my invention.

' When, herein and in the appended claims, it is stated that thealuminumis made the anode, it is to be understood that the use of eitherdirect or alternating current is contemplated and that solong as thealuminum is the anode with direct current or an electrode withalternating current, the anodic oxide coating will be produced.

I claim:

l. The process of producing oxide-coated aluminum which comprises makingthe article to be coated an anode in an electrolytic cell, theelectrolyte of which contains in solution 0.5 to '70 per cent sulfuricacid and a dibasic organic acid of the group consisting of oxalic,malic, ma-

lonic,'maleie and succinic acids in the range of -0.5 per cent to thesaturation limit.

2.'The process of producing oxide-coated aluminum which'comprises makingthe article to be coated an anode in an electrolytic cell, theelectrolyte of which contains in solution 0.5 to 15 per cent sulfuricacid and 0.5 to 9.0 per cent oxalic acid.

5. A process of producing oxide-coated alumi- 1 num articles whichcomprises making the article to be coated an anode in an electrolyticcell, the electrolyte of which contains in solution about 0.5 per centsulfuric acid and about 3 per cent oxalic acid.

6. A process of producing oxide-coated aluminum which comprises makingthe article to be coated an anode in an electrolytic cell, theelectrolyte of which contains in solution about 3 per cent of sulfuricacid and about 3 per cent of malonic acid.

7. The process of producing oxide-coated aluminum which comprises makingthe article to be coated an anode in an electrolytic cell, theelectrolyte of which contains in solution about 3 per cent sulfuric acidand about 3 per cent malic acid.

8. The process of producingoxide-coated aluminum which comprises makingthe article to be coated an anode in an electrolytic cell, theelectrolyte of which contains insolution 0.5 to

70 per cent sulfuric acid and oxalic acid in the rarge of 0.5 per centto the saturation limit.

HAROLD K. WORK.

- -.CERTIFIGATE 0F GORREG.TION.

Patent No. l,965, 682. July. lfl,1.}1934.

' HAROLD K. ,iwonlt.

It is hereby certified that error appears in the printed specificationof the above numbered patent requiring correction is follows: Page 1,for the patent number "I, 695, 682" in the upper right-hand corner read1,965,682: and that the said Letters Patent should be read with thiscorrection therein that the same may conform to the record of the casein the Patent Office.

e. Signed and sealedthis 25th day or September, A. I); 1934,

Leslie Frazer (Seal) Acting Commissioner of Patents-

